MAGNETOS FOR WHICH A LONG LIFE IS CLAIMED.
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An Instrument Specially Designed for Strenuous Service on London Buses.
IN THE course of our recent inspection of the NS-type bus chassis at the works of the Associated Equipment Co., Ltd., we heard very good accounts of the magneto specially designed for the London General Omnibus Co, and made at the Peel Conner Telephone Works in
Coventry. In this machine there has been incorporated all those refinements of detail which the L.G.O. maintenance staff have been able to suggest in the light of their long experience in the maintenance of magnetos on the London bus service.
The instrument in question is claimed to represent a perfected example of standard practice, each detail which has, given trouble in the past having been very carefully analyzed, reconstructed and tested so that the Machine is now practically of uniform strength and reli ability throughout its entire construction. The parts are finished inside and outside with the precision of a fine watch, and the materials have been very, carefully chosen for the duties which', they have to perform. For instance, .the cantact breaker materials are now higNly ductile, so that the parts will bend before breaking, whilst all the ebonite used in the machine is tested in oil at 40,000 volts and again in air at 15,000 volts.
The armature windings are specially inaulated and baked in ovens of a particular type for two days before being assembled in the machine. The contact. breaker rocker-arm bush has been subjected to a special vacuum-impregnating process which entirely removes moisture from the fibre and replaces it with what is known as al hygroscopic fibre-filling substance. • The contacts are made of the hardest possible platinum iridium alloy, and cost more than double the price of the average platinum iridium contacts. The distributor gear is made of a ring of gun-metal which is riveted to a • Bakelite disc and rotates in two annular ball bearings enclosed in a removable un-meta1 bracket. This method of constructing the gearwheel insulates the distributor ball bearings so that no pitting can occur. Furthermore, the Bakelite disc damps out vibration and is nonwarping.
The user is not required to do any lubricaeng of the machine in service, as all bail bearings are picked with a special high-temperature ball-bearing grease, which does noti melt until the temperature is well above boiling-point. The driving-end bearing is provided with a large oil-retaining receiver filled with oil
saturated felt, which acts as an emergency lubricator should the driving-end bearing be subjected to maltreatment due to any possible want of alignment in the magneto coupling.
Numbers of these machines have been put to most strenuous tests, and one in particular has just completed a 1,300. hour run at 3,000 r.p.m. This machine has been enclosed. in an asbestos-lined cabinet in which a temperature of 180° F. was maintained during the day-time when the machine 'was running. At night, the heat was cut off and the cabinet was filled with water-soaked cloths so that the machine (which was stopped), as it cooled down from the test run, would draw moisture into its interior and thus affect any hearings or insulation that it was possible,to.hurt by moisture. So far as the insulation was concerned, nothing whatever occurred to interfere with the operation of the machine, nor did the contact-breaker bush give any indication of sticking,
Taking the armature first, the steelstrengthening sleeve fitted to its contactbreaker end is designed to prevent the spreading of the metal in the event of the contact breaker being screwed home too' tightly. The condeneers are made of the highest possible grade ruby mica, while the condenser clips are composed of duetile metal to avoid breakages due to vibration. The pole pieces of the tungsten-steel magnets are provided with ex tension horns to prevent plug-burning by the magneto at higji speeds, and to produce aeuniform spark over the full range of advance and retard when this control is fitted. The driving end-plate is made from gun-metal specially strengthened to withstand the possibility of the misalignment of the magneto coupling and also to resist the effects of prizing off the coupling member from the armature shaft. The special emergency oil well provides an extreleupply of lubricant (owing to the melting of the grease
. which hitherto sealed the channel), when the bearing is specially heavify stressen.
The distributor is made of a new material and is marhined all over so as to ensure its outer face being free from purities left from the mould. The contact studs and knobs are sunk below the surface of the distributer, the breaking of the studs and shrouds being thus avoided. A reinforced gauze window is fitted. to serve as an exhaust for the nitrous gases formed by brush discharges in the distributor, the air entering through the brass end-plate, passing along the armature and up the gear drive to the distributor, from which it is ejected by the rotating action of the distributor brush. The brush holder is designed to give the maximum creepage surface, all metal parts being plated and sealed to prevent the generation of nitrous gases and the gradual destruction of these parts.
The ebonite partsare made from a special Para rubber, and are subjected to 40,000-volt test under oil before being assembled in the instrument. Every magneto is put through a most exhaustive test, discharging at 7,500 volts for six hours, at the end of which time it is completely dismantled, examined, reassembled and tested for a low-speed performance. Finally, every magneto must give a 7,500-volt spark at. annroximately 48 rpm., a perfarmance which precluder the necessity for an impulse starter.